(Deoxy)nucleosides are glycosylamines consisting of a base like a purine or a pyrimidine bound to a ribose or deoxyribose sugar, the latter being cyclic pentoses. Examples of these include cytidine, uridine, adenosine, guanosine, thymidine, and inosine. Nucleoside analogues are extensively used as antiviral and anticancer agents because of their ability to act as reverse transcriptase inhibitors or chain terminators in RNA or DNA synthesis [1].
Chemical synthesis of nucleoside analogues has been achieved stereoselectively but using expensive or polluting reagents [2] and involving multistage processes that can be time consuming. Biocatalytic procedures offer a good alternative to the chemical synthesis of nucleosides because biocatalyzed reactions are regio- and stereoselective and allow the decrease of by-products content. Of particular interest within the biocatalytic procedures is the enzymatic transglycosylation between a sugar-donating nucleoside and an acceptor base by means of enzymes that catalyse the general reversible reactions [3] as depicted in FIGS. 1 and 2.
Nucleoside phosphorylases are transferases widely distributed in mammalian cells and bacteria and play a central role in the nucleoside metabolism salvage pathway. They have a dual functionality. On the one hand, they catalyse the reversible cleavage of the glycosidic bond of ribo- or deoxyribo nucleosides in the presence of inorganic phosphate in order to generate the base and ribose- or deoxyribose-1-phosphate. These enzymatic reactions employing the purine nucleoside phosphorylases and the pyrimidine nucleoside phosphorylases are shown in FIG. 1. On the other hand, these enzymes catalyse phosphate-dependent pentose transfer between purine or pyrimidine bases and nucleosides, i.e. transglycosylation reactions, to produce nucleosides with differing bases. FIG. 2 shows an example of a one-pot synthesis using nucleoside phosphorylases.
When the pyrimidine and purine nucleoside phosphorylases are used in combination, it is possible to transfer the sugar from a donor pyrimidine nucleoside to a purine or pyrimidine acceptor base as well as from a donor purine nucleoside to a pyrimidine or purine acceptor base, depending on the starting materials used [4]. As a consequence, nucleoside phosphorylases from different sources, mainly bacterial, have been exploited as tools for the enzymatic synthesis of nucleoside analogues.
In nature these enzymes have been described in various microbial strains, particularly in thermophilic bacteria (i.e. bacteria thriving at temperatures between 45° C. and 80° C.), which have been used as sources of nucleoside phosphorylases in numerous works for obtaining modified nucleosides by enzymatic transglycosylation. However, although in these studies the target products yields were sufficiently high, the amount or ratio of the enzymatic activities necessary for transglycosylation was non-optimal [5]. They required either a considerable extension in the reaction time (up to several days) or an increase in the used bacterial biomass to reach the necessary transformation depth.
Besides, when developing a transglycosylation process another problem arises: the difficult solubilization of large amounts of substrates and products, many of them poorly soluble in aqueous medium at room temperature. Although this problem could be solved using higher temperatures, it requires enzymes sufficiently stable in these harder reaction conditions.
The Archaea are a group of single-celled microorganisms that are one of the three domains of life; the others being Bacteria and Eukarya. They were formerly called Archaebacteria under the taxon Bacteria, but now are considered separate and distinct. The archaeal domain is currently divided into two major phyla, the Euryarchaeota and Crenarchaeota. The Euryarchaeota includes a mixture of methanogens, extreme halophiles, thermoacidophiles, and a few hyperthermophiles. By contrast, the Crenarchaeota includes only hyperthermophiles. Hyperthermophiles are those organisms that thrive in extremely hot environments, from 60° C. upwards, optimally above 80° C.
Cacciapuoti et al. [6-8] describe two purine nucleoside phosphorylases (PNPases) from hyperthermophilic Archaea, in particular it discloses the enzymes 5′-deoxy-5′-methylthioadenosine phosphorylase II (SsMTAPII, EC 2.4.2.28) from Sulfolobus solfataricus, and purine nucleoside phosphorylase (PfPNP) from Pyrococcus furiosus. The Pyrococcus furiosus enzyme was firstly annotated as MTAPII but renamed to PNP as it is unable to cleave methylthioadenosine. Sulfolobus solfataricus belongs to the Crenarchaeota, while Pyrococcus furiosus belongs to the Euryarchaeota. The EC code above is the conventional enzyme nomenclature provided by the International Union of Biochemistry and Molecular Biology that classifies enzymes by the reactions they catalyse.
Most enzymes characterized from hyperthermophiles are optimally active at temperatures close to the host organism's optimal growth temperature. When cloned and expressed in mesophilic hosts like Escherichia coli, hyperthermophilic enzymes usually retain their thermal properties. Sometimes the enzymes are optimally active at temperatures far above the host organism's optimum growth temperature [9]. Other times enzymes have been described to be optimally active at 10° C. to 20° C. below the organism's optimum growth temperature [10-11]. However, the Sulfolobus solfataricus 5′-methylthioadenosine phosphorylase (a hexameric enzyme containing six intersubunit disulfide bridges), when expressed in a mesophilic host, forms incorrect disulfide bridges and is less stable and less thermophilic than the native enzyme [12].
The Thermoprotei are a hyperthermophilic class of the Crenarchaeota. From the genomes sequenced and available for the Archaea Thermoprotei class, only three sequences for purine-nucleoside phosphorylase (EC 2.4.2.1) and only three sequences for uridine phosphorylase (EC 2.4.2.3), were found. These six proteins have been entered, respectively, in UniProtKB/TrEMBL with the accession numbers: A1RW90 (A1RW90_THEPD), for the hypothetical protein from Thermofilum pendens (strain Hrk 5); Q97Y30 (Q97Y30_SULSO), for the hypothetical protein from Sulfolobus solfataricus; A3DME1 (A3DME1_STAMF), for the hypothetical protein from Staphylothermus marinus (strain ATCC 43588/DSM 3639/F1); Q9YA34 (Q9YA34_AERPE), for the hypothetical protein from Aeropyrum pernix; A2BJ06 (A2BJ06_HYPBU) for the hypothetical protein from Hyperthermus butylicus (strain DSM 5456/JCM 9403); and D9PZN7 (D9PZN7_ACIS3) for the hypothetical protein from Acidilobus saccharovorans (strain DSM 16705/VKM B-2471/345-15). All these sequences were under the annotation status of unreviewed, which means that their presence in the Archaea has only been verified by computer.
Even though many genes can be successfully expressed in Escherichia coli at high yields, several proteins from hyperthermophiles are poorly or not at all expressed, partially due to the usage of rare codons. Indeed, and to the best of our knowledge, no party was yet successful in expressing any of the mentioned genes above.
In view of the prejudices above, in view of the technical difficulties, the inventors unexpectedly were able to prepare viable recombinant vectors and importantly, obtain recombinant phosphorylases that were optimally active at temperatures higher than 60° C. The thermostable and chemically stable catalysts of the present invention are a purine nucleoside phosphorylase (PNPase, E.C. 2.4.2.1), and a uridine phosphorylase (UPase, E.C. 2.4.2.3), originating from the Archaea Thermoprotei class, wherein the PNPase is from Sulfolobus solfataricus (SEQ ID NO. 7) and the UPase is from Aeropyrum pernix (SEQ ID NO. 8).
In particular, it has been surprisingly found that the recombinant nucleoside phosphorylases derived from the hyperthermophilic Thermoprotei have unique structure-function properties like enhanced thermostability, high catalytic efficiency, and optimal enzymatic activities at temperatures near or above 100° C. These recombinant enzymes can advantageously be used for transglycosylation reactions, in the form of cell lysate and in the form of crude or purified extracts, for industrial production of natural and modified nucleoside analogues. They are in particular versatile since they can catalyze transglycosylations in aqueous media, in organic solvents, at temperatures between 60° C. and 120° C., or in a combination of these parameters, allowing the preparation of many and diverse types of nucleosides at acceptable production yields, reaction times, and employing economical amounts of the enzymes. Importantly, the biocatalysts described in the present invention can be used for bioconversion reactions that require the presence of organic solvents, temperatures above 60° C., or both, in order to solubilize the substrates or the reaction products. These phosphorylases are ideal in reactions with water-insoluble substrates. Another advantage of these phosphorylases resides in their organic solvent tolerance, and in that they can be reused for several reaction cycles.
More advantageously, the invention offers a combination of Thermoprotei nucleoside phosphorylases that is useful for one-pot synthesis of nucleosides. The enzymes can be used to produce natural or analog nucleosides in a one-step (one-pot) or two-step synthetic methods. In the one-step synthesis, a pyrimidine nucleoside phosphorylase and a purine nucleoside phosphorylase are used in the same batch in order to change the base linked to the sugar by another one of choice. In the two step, a pyrimidine nucleoside phosphorylase is used for the liberation of the sugar of a pyrimidine nucleoside, and then, the 1-phosphate-sugar is isolated and later on, in another vessel, a purine base is linked to the sugar using a purine nucleoside phosphorylase.